1. Field of the Invention
This invention relates to a means and method for protecting the wetted parts of pumps that are used to meter air- or moisture-sensitive liquids. The means and method of the invention protect such pumps from corrosion and degradation that can result when the liquids being pumped react with air or moisture to form solid particles.
2. Description of the Related Art.
Recently many refractory materials have been identified as having unique materials properties. The recently discovered high temperature superconducting (HTSC) materials include YBa.sub.2 Cu.sub.3 O.sub.x, wherein x is from about 6 to 7.3, BiSrCaCuO, and TlBaCaCuO. Barium titanate, BaTiO.sub.3, and barium strontium titanate, Ba.sub.x Sr.sub.1-x TiO.sub.3, have been identified as ferroelectric and photonic materials with unique and potentially very useful properties. Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 is a photonic material whose index of refraction changes as a function of electric field and also as a function of the intensity of light upon it. Lead zirconate titanate, PbZr.sub.1-x Ti.sub.x O.sub.3, is a ferroelectric material whose properties are very interesting. The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are materials that are useful for scintillation detecting and coating of optical fibers. Refractory oxides such as Ta.sub.2 O.sub.5 are seeing expanded use in the microelectronics industry; Ta.sub.2 O.sub.5 is envisioned as a thin-film capacitor material whose use may enable higher density memory devices to be fabricated.
Many of the potential application of these materials require their use in thin film, coating, or layer form. The films or layers may also be advantageously epitaxially related to the substrate upon which they are formed. Applications in which the refractory materials may need to be deposited in film or layer form include integrated circuits, switches, radiation detectors, thin film capacitors, holographic storage media, and various other microelectronic devices.
Chemical vapor deposition (CVD) is a particularly attractive method for forming these layers because it is readily scaled up to production runs and because the electronic industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming the thin films by CVD will permit the integration of these materials into existing device production technologies. CVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having close crystal structures.
CVD requires that the element source reagents must be sufficiently volatile to permit gas phase transport into the deposition reactor. The element source reagent must decompose in the reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved if the reagent can be delivered into the reactor in a controllable fashion. In addition, the reagents must not be so chemically stable that they do not react in the deposition chamber.
Thus a desirable CVD reagent is fairly reactive and volatile. Unfortunately, for many of the refractive materials described above, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are elements, such as the Group II metals barium, calcium, or strontium, or early transition metals zirconium or hafnium, for which no volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it will be very difficult to control the stoichiometry of the deposited films.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven problematic because of problems of premature decomposition or stoichiometry control.
The problem of controlled delivery of CVD reagents into deposition reactors was addressed by the inventors in U.S. patent application Ser. No. 07/807,807, now U.S. Pat. No. 5,204,314, patented Apr. 20, 1993, which is a continuation of U.S. patent application Ser. No. 07/549,389, "Method for Delivering an Involatile Reagent in Vapor Form to a CVD Reactor," now abandoned, and further elaborated in U.S. patent application Ser. No. 07/927,134, "Apparatus and Method for Delivery of Involatile Reagents," which hereby are incorporated herein by reference and is now abandoned. As described and claimed in these patents, the delivery of reagents into the deposition chamber in vapor form is accomplished by providing the reagent in a liquid form, neat or solution, and flowing the reagent liquid onto a flash vaporization matrix structure which is heated to a temperature sufficient to flash vaporize the reagent source liquid. A carrier gas may optionally be flowed by the flash vaporization matrix structure to form a carrier gas mixture containing the flash vaporized reagent source liquid.
The means for flowing the reagent liquid onto the flash vaporization matrix may be any suitable liquid pumping means, such as a positive displacement liquid pump. In practice, the method chosen for pumping the liquid is often a piston pump.
Serious problems of pump particle generation and hence plugging of orifices and degradation of seals are encountered when air- or moisture-sensitive liquids are metered by piston pumps that have moving, wetted parts. For example, when the metering piston's wetted surfaces are exposed to air, reactions between the liquid being pumped and air or moisture can occur that produce oxidic particles. These particles erode the piston seals, leading to pump breakdown.
A related problem is the degradation of pump seals that can result when the reagent being pumped is a solid dissolved in a relatively volatile liquid solvent. The solvent evaporates, leaving behind the solid which abrades the seal.
Many reactive liquids that are used as source reagents in processes for film or layer deposition have caused problems in pumping. Some of these compounds are readily hydrolyzed by moisture in the air, such as tantalum ethoxide, tetraethyl orthosilicate (TEOS), other metal alkoxide compounds such as zirconium tetra-tert-butoxide, and metal amide reagents such as tetrakis(dialkylamido)titanium compounds. These moisture-sensitive compounds react to form oxide particles that are especially hard on the piston seals. Other compounds used in deposition processes are highly air-sensitive. Examples include the aluminum source reagents such as tri-isobutylaluminum and trimethylamine alane (a solid which may be used in solution in a solvent which is chemically inert to the aluminum reagent, such as hexane), other Group III reagents such as trimethylgallium, and some Group V reagents such as trialkylantimony compounds. Such compounds react with oxygen, likewise to form destructive oxide particles.
For example, when a dual piston metering pump was used to deliver tantalum pentaethoxide, a moisture-sensitive liquid, tantalum oxide built up on the pistons after tens of hours which eroded the piston seals and eventually stopped any piston movement.
This problem with pumping reactive liquids has hindered the usage of liquid delivery systems of all sorts, including but not limited to the type described and claimed in U.S. patent applications Ser. Nos. 07/807,807 and 07/927,134. In reactive liquids pumping systems that do not employ the inert purge blanket system of the present invention, maintenance becomes a problem. Such pumping systems have a shorter mean time to failure and frequent downtime for maintenance steps such as seal replacement.
Similar pumping problems have been encountered in other systems. In most cases, the solution proposed to address attack on piston seals and other wetted parts has been to coat the parts with a chemically resistant coating, for example Teflon.RTM. coatings or coatings of other inert polymeric material(s). This approach, however, does not address the problems created by oxide particle formation as described above. While the pump parts may thereby be protected from chemical attack, the particles still have the potential to physically abrade moving parts, clog orifices, and score the chemically resistant coating(s).
U.S. Pat. No. 3,5 16,760 describes a method to protect a piston pump conveying a corrosive reaction mixture, at least one constituent of which is a liquid which does not corrode the material in the stuffing box. A suitable amount of the noncorrosive liquid is injected into an annular gap surrounding the piston, under pressure sufficient to prevent the corrosive mixture from reaching the stuffing box. In the manufacture of urea, the packing material is protected from corrosive attack by the carbamate intermediate by injecting liquid ammonia into the annular gap. As ammonia enters the carbamate mixture, this procedure provides a means for returning ammonia which has been lost from the mixture, thus increasing the yield of carbamate as well as preserving the packing in the stuffing box. The ammonia acts as a scavenger, since it is one of the reactants in the process. This approach is not broadly applicable, since not all air- or moisture-sensitive liquids being pumped contain a noncorrosive component, and indeed not all liquids being pumped are mixtures.
Accordingly, it is an object of the present invention to provide a means and method for protecting the moving parts of pumps used to deliver air- and moisture-sensitive liquids by which these previous obstacles are overcome.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.